Slab Edge Insulation and Heat Loss
How does slab edge insulation affect heat loss through a monolithic, slab-on-grade floor? Does the earth below the slab essentially become part of the building envelope? Or will the slab still lose a lot of heat through the floor in the winter? (while hopefully gaining cooling potential in the summer)
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What climate zone are you in? Are you talking about a slab with sub-slab insulation? Are you talking about a raised slab with a frostwall or a thickened-edge slab with exterior insulation?
Climate Zone 4A. No sub-slab insulation, just a thickened edge slab with exterior insulation. The first image in this article: https://www.greenbuildingadvisor.com/article/different-types-of-insulated-slabs
According to ChatGPT, the heating loss is Q = U x A x DD x HSL x P where:
U = 0.4828 Btu/hr-ft²-°F (heat loss coefficient)
A = 121.5 sq.ft (floor area)
P = 45 ft (perimeter of the slab)
DD = 3963 (annual heating degree days)
HSL = 7/12 (heating season length)
Which gives me a heat loss of 1788.421 KW per year through the floor or ~$250 per year in heating? Sounds sort of right to me, but it doesn't account for the heat loss through the walls, windows, doors, etc. so maybe it's a bit high?
This article https://www.greenbuildingadvisor.com/article/getting-slab-edge-insulation-right though calculates a heating cost of $336 per year for a 1,835 sq ft slab. If we reduce those numbers to the size of my 121.5 sq ft slab, we'd only be looking at $23 in yearly heating. Something must not be adding up.